1. Technical Field
The present disclosure relates to the field of digital image sensors, and in particular to the field of high dynamic range methods for such sensors.
2. Discussion of the Related Art
Digital image sensing based upon solid state technology is well known, the two most common types of image sensors currently being charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors. Digital image sensors are incorporated within a wide variety of devices throughout the consumer, industrial and defence sectors among others.
An image sensor is a device comprising one or more radiation sensitive elements having an electrical property that changes when radiation is incident upon them, together with circuitry for converting the changed electrical property into a signal. As an example, an image sensor may comprise a photodetector that generates a charge when radiation is incident upon it. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths, or other neighbouring wavelength ranges, such as infra red or ultra violet for example. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
Typically, more than one radiation sensitive element will be provided in an array. The term pixel is used as a shorthand for picture element. In the context of a digital image sensor, a pixel refers to that portion of the image sensor that contributes one value representative of the radiation intensity at that point on the array. These pixel values are combined to reproduce a scene that is to be imaged by the sensor. A plurality of pixel values can be referred to collectively as image data. Pixels are usually formed on and/or within a semiconductor substrate. In fact, the radiation sensitive element comprises only a part of the pixel, and only part of the pixel's surface area (the proportion of the pixel area that the radiation sensitive element takes up is known as the fill factor). Other parts of the pixel are taken up by metalization such as transistor gates and so on. Other image sensor components, such as readout electronics, analog to digital conversion circuitry and so on may be provided at least partially as part of each pixel, depending on the pixel architecture.
One of the most important characteristics of any image sensor is its dynamic range, that is, the ratio between the minimum and the maximum signal that can be successfully reproduced by the image sensor. There are various fields in which a high or very high dynamic range is required.
One such device where a wide dynamic range is required is a biosensor. In a biosensor, each pixel is exposed to a substance suspected of containing target diseases/chemicals/proteins etc. which is then treated with a chemical that reacts specifically with the target. This reaction produces light which is then detected by the biosensor. Different targets and their corresponding chemicals can be put on different pixels on the biosensor array, such that the system can analyze a wide range of samples at once. However, it is often the situation that each pixel will receive vastly different light levels. Some pixels saturate in fractions of seconds, while others may take up to 30 seconds to saturate.
It would be desirable, therefore, to provide readout circuitry able to analyze all these signals, therefore greatly increasing the pixel array's dynamic range.